A. Arginine vasopressin
Arginine vasopressin (AVP), a neurohormone also known as anti-diuretic hormone, is characterized by a nine amino acid, partially cyclic structure. AVP has been reported to be associated in serum with a binding protein, called neurophysin [Brain Peptides, (D. T. Krieger et al, eds.), John Wiley & Sons, New York, pp. 598-611 (1983)]. AVP is secreted from two major locations in the brain, from hypothalamic parvicellular neurons in the paraventricular nucleus, which also produce corticotropin releasing factor (CRF), and from magnocellular neurons in the supraoptic and paraventricular nuclei [F. A. Antoni, in Frontiers in Neuroendocrinology, 14(2):76-122 (1993)]. CRF is known to synergize with AVP to stimulate ACTH release.
It has been demonstrated that, following the application of chronic stress paradigms in laboratory animals, there is an increase in the level of AVP in the paraventricular nucleus of the hypothalamus. This AVP level is disproportionately large compared with the increase observed in CRF levels [Antoni, cited above; D. C. De Goeij et al, Endocrinol., 131:847 (1992)]. Following chronic stress, AVP levels within CRF-containing neurons within the paraventricular nucleus of the hypothalamus of laboratory animals have been reported to increase by over 8 fold, while the increase in CRF levels was 1.5 fold. This disproportionate increase in the level of AVP in this portion of the hypothalamus as compared with CRF has also been observed to be maintained at the level of the median eminence, the terminal bed from which AVP and CRF are released to stimulate ACTH secretion from the pituitary.
Previous attempts have been made to correlate altered plasma levels of both CRF and AVP with psychiatric disorders, such as depression, in humans. However, attempts to establish a diagnostic evaluation of such conditions by measuring blood levels of AVP have been unsuccessful. Such efforts have reported either no difference in AVP levels between depressed patients and normal controls, or a decrease in cerebrospinal fluid (CSF) AVP levels caused by depression. For example, P. S. Sorensen et al., J. Neurol. Neurosurg. Psych., 48:50-57 (1985) found no difference in AVP levels in the plasma of depressed patients vs. normal controls.
In a study by P. W. Gold et al., Psychopharmacol. Bull., 19:426-431 (1983), decreases in AVP below the control levels in CSF were found in non-psychotic depressed patients. Other researchers have attempted to correlate AVP levels in CSF and in plasma for depressed patients and have found a decrease in CSF AVP, but no difference in plasma AVP [A. Gjerris et al, Brit. J. Psychiatry, 147:696-701 (1985)]. A more recent study of AVP levels in the CSF of depressed patients vs. normal controls also revealed a decrease in AVP levels for such patients [A. Gjerris, Acta Psychiatr. Scand., 78, Suppl. 345:21-24 (1988)]. To date, no one has ever reported an increase in plasma AVP levels in depression.
Blood levels of AVP have been studied most extensively in relation to water and electrolyte balance in the body. For example, a lack of AVP is associated with diabetes insipidus, which causes a failure to retain fluid by the kidneys and a resultant decrease in electrolytes. This condition is treated by the administration of AVP. There is also a rare syndrome of elevated AVP in blood, referred to as the syndrome of inappropriate antidiuretic hormone secretion (SIADH). See, e.g., S. Hou, "Syndrome of Inappropriate Antidiuretic Hormone Secretion" in Reichlin S., ed., The Neurohypophysis, Plenum Press, New York (1984) pp. 166-189. Occasionally, an excess of AVP has been found associated with certain cancers, such as malignant neoplasias, lymphomas, leukemias, thymomas and mesotheliomas. Inappropriate AVP levels have also been observed in rare cases of pulmonary disorders such as tuberculosis and pneumonia and in central nervous system disorders such as trauma. Abnormal AVP levels have also been noted as a consequence of drugs that enhance AVP release or action, e.g., diuretics.
However, the changes or elevations of AVP blood levels in these relatively rare conditions are accompanied by physiologic symptoms and changes in serum electrolytes, and are thus clearly distinguishable by the context in which such elevation is observed [See, e.g., Textbook of Endocrinology, 7th ed. (Wilson and Foster, eds) 1985 pp. 644-645]. Such increases in AVP have been detected by radioimmunoassay.
B. Thymopoietin
The thymic hormone thymopoietin (TP) has been shown to play a regulatory role in immune, nervous, and endocrine functions and has been isolated from bovine and human thymus. For additional general information on TP, see, also, G. H. Sunshine et al, J. Immunol., 120:1594-1599 (1978); G. Goldstein, Nature, 247:11-14 (1974); D. H. Schlesinger and G. Goldstein, Cell, 5:361-365 (1975); G. Goldstein et al., Lancet 2:256-262 (1975). TP has also been found to be present in brain extracts [R. H. Brown, et al., Brain Research 381:237-243 (1986)].
It has been found that as the thymus involutes with age, thymic hormone levels decrease, which is believed to be related to increased susceptibility to disease in aging [G. Goldstein and I. R. Mackay, The Human Thymus, Wm. Heineman Med. Books Ltd., London (1969)]. Additionally, hypersecretion of TP has been implicated in myasthenia gravis [G. Goldstein, Lancet, 2:1164-1167 (1966)], as being involved in the impairment of signal transmission from nerve to muscle. When this signal is interrupted, the result is generalized weakness.
Previous attempts to measure TP levels by bioassay have suggested various differences in TP levels in different pathological states. However, to date, no one has provided any correlation between affective disorders and TP.
Bioassays used to measure TP are cumbersome, inaccurate and unreliable. [J. J. Twomey, et al., Proc. Natl. Acad. Sci. USA, 74:2541-2545 (1977); V. M. Lewis, et al., J. Clin. Endo. Metab. 47:145-150 (1978); J. J. Twomey, et al., Am. J. Med. 68:377-380 (1980)]. Immunoassays are the preferred format for measuring peptides and proteins in plasma or serum, but prior attempts to develop immunoassays to measure TP have not yielded clinically useful techniques. For example, a displacement radioimmunoassay (RIA) for measuring bovine TP was developed that detected TP concentrations greater than 5 ng/mL in tissue extracts. However this RIA is incapable of measuring TP levels in blood [see, e.g., G. Goldstein, J. Immunol. 117:690-692 (1976)].
The sensitivity of the TP RIA was subsequently increased to 20 picograms (pg) [see, e.g., P. J. Lisi et al, Clin. Chim. Acta, 107:111-119 (1980)] using "human serum-based standards" and rabbit antisera. However, this assay has not proved effective or reproducible in practice. In addition, the present inventors have found that 20 picograms sensitivity is too poor to detect human blood levels of TP. A sandwich enzyme-linked immunoassay (ELISA) was later developed for bovine TP using a combination of polyclonal and monoclonal antibodies [A. Fuccello et al, Arch. Biochem. Biophys., 228:292-298 (1984)]. Although the assay provided specificity in distinguishing bovine TP from bovine splenin, it proved ineffective in measuring TP in humans.
Direct measurement of TP in human plasma or serum has not been accurately reported. Possible reasons for this problem may include aggregation of TP with either itself or other blood proteins, complexing of TP with specific proteins, and too low a concentration of TP to be detected by standard assay methods.
There remains a need in the art for the development of reliable methods based on blood levels of AVP and/or TP, which enable the laboratory diagnosis or confirmation of affective disorders, such as depression and/or anxiety, in humans.